The processing and purification of metal sulfide containing ores involves various unit operations, including, without limitations, pre-leach crushing, pre-leach grinding, and pre-leach froth flotation. In the pre-leach froth flotation process, surface-active reagents are used to selectively alter the wetting characteristics of sulfide mineral surfaces to promote their separation from gangue minerals. The surfactant-modified particles are separated and recovered by virtue of their selective partitioning from the mineral slurry to a collected froth. Various types of froth flotation reagents are commonly used in mineral separations, including collectors, frothers, activators and depressants. When the mineral-containing pulp is aerated, the surface-modified particles have a tendency to attach to the air bubbles, and rise by buoyancy to produce a mineralized froth which is concentrated atop the surface of the agitated, mineral pulp. This froth is collected as a concentrate which is then oxidatively-leached.
In the hydrometallurgical processing of copper sulfide concentrates, copper concentrate is typically dispersed in an acidic ferric sulfate leach liquor to bring about dissolution of the copper contained in the mineral particles. The leach process produces a pregnant leach solution (PLS) which is then treated by a solvent extraction (SX) process to separate and recover the dissolved copper. The SX process is followed by electrowinning to produce high-purity copper cathodes.
In some prior art leach processes (see, for example, U.S. Pat. No. 5,993,635), a flotation concentrate is initially subjected to ultra-fine grinding, followed directly by oxidative leaching under atmospheric conditions. In these methods, the copper is dissolved from the copper-bearing minerals at temperatures below the boiling point of water. Although there may be localized, transient heating to temperatures of 100° C. or slightly higher, due to exothermic chemical reactions, the pulp temperature is for the most part limited due to the fact that the system is at atmospheric pressure.
An oxidizing agent, such as ferric ion is commonly used to facilitate the copper dissolution reaction from copper bearing sulfide minerals. During the course of this chemical reaction, the oxidizing agent (i.e., ferric ion) is reduced from the ferric oxidation state to the ferrous oxidation state. To continue the process until the majority of the copper is recovered from the mineral particles, oxygen or air is sparged into the stirred reactor to continuously oxidize the generated ferrous ion back to its +3 oxidation state. In the case of chalcopyrite dissolution, ferric ions are believed to promote the leaching of copper via the following stoichiometry:CuFeS2+4Fe3+=Cu2++5Fe2++2S∘
Simultaneous regeneration of the ferric oxidant and maintenance of electro-neutrality is believed to proceed via the following reaction:4Fe2++O2+4H+=4Fe3++2H2O
Consequently, acid is consumed during the electrochemical leaching of chalcopyrite. Similar reactions in which ferric ion acts as an oxidant are known for the leaching of a variety of metal sulfides, including copper, zinc, iron, manganese, nickel, cobalt, etc.
During the course of the atmospheric leach process, crystalline, elemental sulfur (S∘) is produced as a reaction product by virtue of the temperatures and oxygen pressures employed. Because the temperatures involved are below the melt temperature of elemental sulfur, the sulfur appears predominantly as a crystalline phase on the surface of the copper-bearing mineral particles being leached.
During the initial stages of the leach process, the surfaces of the copper-bearing mineral particles are amphiphilic due to the presence of hydrophobic sulfur and residual flotation reagents. As the leach process progresses, the accumulation of elemental sulfur causes the copper-bearing particles to become progressively more hydrophobic. During the early stages of the leach process, the combination of fine particle size and the amphiphilic nature of the particle surfaces leads to the formation of a stable froth. During the later stages of the leach process, the accumulated elemental sulfur on these particles can act as a physical barrier, and simultaneously promotes particle-particle agglomeration, thereby inhibiting (i.e., passivates) continued copper dissolution from the mineral particles.